This invention relates to light scanners, and, more particularly, to an improved light scanner and light scanning system with wobble correction.
There are various important uses for light scanners which scan a collimated light beam, often a laser beam, in a line pattern across a target. This is commonly achieved by using a rotating optical scanner which repetitively scans the laser beam to produce a line scan. Among the applications for such scanners are copying and facsimile equipment, laser printers, character recognition equipment, and specialized graphics equipment. The accuracy of the scan is generally an important factor in equipment performance. Unfortunately, undesired angular perturbations in the cross-scan direction, called "wobble", result in a scanned line that varies from its intended straight or repetitive path. The wobble can be due to angular differences between multiple elements (e.g. facets) which form the repetitive scans, and/or due to pseudo-random angular shaft errors, often resulting from bearing non-uniformities. Even when the optical scanner is a single-facet device in which periodic differences may be nulled on a once-per-revolution basis, the residual pseudo-random errors can cause significant misplacements of the scan lines in the image area.
There have been various prior art approaches to reducing wobble. One method is the use of anamorphic (usually cylindrical) optics for wobble reduction. This is accomplished by first compressing the height of the illuminating beam incident upon the scanner in the cross-scan direction only (usually with an input cylinder) and then, after the beam is deflected, re-expanding the beam in the cross-scan direction only (usually with an output cylinder) as it propagates through the balance of the system optics, before forming the final focused image. Cross-scan angular error is reduced by the ratio of the beam compression (or re-expansion). For further description, reference can be made to the book by L. Beiser, "Holographic Scanning", John Wiley, (1988). However, in the case of the single facet scanner whose mirror is mounted at 45.degree. to the rotating axis while the input beam is paraxial, this correction method fails because the compressed component of illumination is intercepted at different angles as the facet rotates, distorting and skewing its output during scan.
Another technique of wobble reduction utilizes a single linear (transmission) holographic grating mounted to a shaft at 45.degree. such that is acts as a 45.degree. mirror. It reduces cross-scan errors when the input and output beams are related to the grating normal by the Bragg angle (in this case, 45.degree.). In addition to the specialized technology necessary for making such gratings, the angular sensitivity of the gratings at different wavelengths, and the diffraction efficiency sensitivity to different polarization angles during rotation are limiting factors.
U.S. Pat. No.s 4,475,787 and 4,606,601 disclose methods and apparatus for reducing wobble using double reflection. U.S. Pat. 4,475,787 shows a pentaprism or pentamirror mounted to the rotating shaft to act as a monogon (single facet scanner). Double-reflection nulls the cross-scan error; that is, the error of the first reflection is nulled by an equal and opposite second reflection. Limitations of this approach are the significant bulk and unbalanced mass of the penta components, requiring further counter-balancing to achieve stable rotation. Also, the components form a relatively large optical assembly which can impose packaging and cost constraints. A variation described in U.S. Pat. No. 4,606,601 operates also by double reflection; however, only as a right angle roof mirror or prism upon an input beam which is incident normal to the axis.
It is among the objects of this invention to provide a light scanner which reduces wobble, but without the limitations or drawbacks of prior art wobble correction schemes.